This invention relates generally to optical circuits and networks, and more particularly, to variable optical delays.
Variable optical delays have potential applications in both optical data networks and optical logic circuits. The applications involve synchronizing internal components of such networks and logic circuits to external data streams and other internal components, respectively. Synchronizing entails changing the arrival times of optical signals.
One potential application of such delays is the construction of packet-switched optical networks. Packet-switched networks need to resynchronize receivers on a pack-by-packet basis. The need for packet-by-packet resynchronization may be met by variable time delays produced by either delay lines or clock recovery techniques.
The prior art includes several types of variable optical delay lines. Some such lines use either a stepping motor or a piezo-electric transducer to mechanically change the length of an optical fiber or a gap, carrying the arriving signal. Other delay lines use an acousto-optic modulator or another type of beam scanning crystal to convert changes in arriving beam angles into variable delays. These types of delay lines are typically characterized by response times on the order of milliseconds or longer.
The prior art also includes techniques for varying the phase of an optical clock. One clock recovery technique uses electro-optical phase locked loops. Another clock recovery technique uses injection locking of a receiver""s optical clock to the data stream. Both of these techniques have response times in the millisecond range.
In one aspect, the invention provides a method to introduce variable time offsets into a stream of optical pulses. The method includes receiving a plurality of coherent optical pulses, receiving a plurality of control signals, and forming a coherent pulse array (CPA) from each pulse in response to one of the received control signals. Temporal spacings between pulses of each CPA are responsive to the associated one of the received control signals. The method further includes transmitting each pulse through a dispersive optical medium. The act of transmitting makes pulses of each CPA overlap to form an interference pattern.
In some embodiments, the method further includes sending each interference pattern through an intensity discriminator to pass a peak thereof.
In some embodiments, the act of forming a CPA for each pulse further includes splitting each received pulse into a plurality of pulses, and delaying at least one of the pulses. The act of delaying includes propagating the one of the pulses and the associated one of the control signals in a nonlinear optical media.
Some embodiments further filter the associated one of the control pulses from the nonlinear medium. Other embodiments propagate the pulses and the associated control signals in opposite directions in the nonlinear medium.
In general, in a second aspect, the invention features a variable temporal grating generator (TGG). The variable TGG includes an amplitude splitter to split a received optical pulse into a plurality of pulses, a plurality of optical waveguides, and a waveguide coupler connected to receive pulses from the optical waveguides. Each waveguide receives one of the pulses from the splitter. At least one of the waveguides has a variable path element. The variable path element has a control terminal and a optical path length responsive to control signals received at the control terminal. The coupler has an output terminal to transmit CPA""s made of the pulses received.
In some embodiments, the variable path element further includes a nonlinear optical medium coupled to receive pulses traveling through the waveguide. The signals received by the control terminal are optical signals. The control terminal transmits a portion of each optical signal to the nonlinear medium.
In some embodiments, the variable TGG further includes an optical waveguide coupled to receive the CPA""s from the output terminal and a high frequency signal generator. The generator sends electrical or optical driving signals to a portion of the optical waveguide. The driving signals vary the index of refraction of the portion of the waveguide.
In general, in a third aspect, the invention features a variable optical delay line. The optical delay line includes a length of dispersive medium and a TGG having an optical input terminal, an optical output terminal and a control terminal. Either the optical input terminal or the optical output terminal couples to one end of the dispersive medium. The TGG generates a CPA at the optical output terminal from each pulse received at the optical input terminal. Temporal spacings of pulses of each CPA are responsive to control signals received at the control terminal. The dispersive medium causes each CPA to produce an interference pattern.
In some embodiments, the dispersive media is a dispersive optical waveguide. The variable TGG may also include an optical clock producing coherent clock pulses. The output terminal of the clock connects either to an end of the waveguide or to the input terminal of the variable TGG. The variable optical delay line may also include an intensity discriminator to receive each interference pattern.
In various embodiments, the variable TGG further includes an amplitude splitter and a plurality of optical waveguides. The splitter splits an optical pulse received from the input terminal into a plurality of pulses. Each waveguide connects to receive one of the pulses from the splitter. At least one of the waveguides includes a variable path element coupling to the control terminal. The variable path element has an optical path length responsive to the control signals. The variable TGG also includes a waveguide coupler connected to receive pulses from the optical waveguides. The waveguide coupler has a second output terminal to transmit a portion of the pulses received.
The variable path element may further include a nonlinear optical medium coupled to receive pulses traveling through the one of the waveguides. The signals received by the control terminal are optical signals. The control terminal is connected to transmit a portion of each optical signal to the nonlinear medium.
In general, in a fourth aspect, the invention features an optical phase locked loop (OPLL). The OPLL includes an optical switch, an optical clock, a dispersive optical waveguide coupled to the optical clock, and a variable TGG having a control terminal. The switch has two input terminals and one output terminal. The variable TGG receives clock pulses from the dispersive waveguide and transmits interference patterns to one input terminal of the optical switch. The output terminal of the switch couples to the control terminal.
In various embodiments, the output terminal of the optical switch transmits optical signals to the control terminal.
In general, in a fifth aspect, the invention features an antenna array. The array includes a plurality of remote antennae and a control system to produce optical control signals. The array includes a plurality of first optical waveguides that receive the signals from the control system. The array also includes a plurality of variable TGG""s and a plurality of second waveguides. Each TGG couples to one of the first waveguides. Each second waveguide connects one of the TGG""s to one of the remote antennae. Each second waveguide produces an interference pattern from a CPA received from the connected TGG.
Other features, and advantages of the invention will be apparent from the following description of the preferred embodiments and the claims.